With the development of dual band focal plane arrays which image both the mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) spectral bands simultaneously at the same image plane, the development of various types of optical systems has been required. In order to keep the same capabilities as a single band system (i.e., MWIR only), it is necessary to develop similar types of optical systems. One useful type of optical system is a continuous zoom lens arrangement which enables FOV changing from the lenses shortest focal length to its longest focal length, and every focal length in between with good optical performance. This has previously been accomplished in both the MWIR and LWIR spectral bands independently. However, a dual band system requires a more complex optical design because of the need to image both spectral bands at the same image plane even though this is not the natural tendency of the optics. These lenses require significant color correction for each focal length. This is very difficult for systems that require significant focal length changes.
Due to the reduced transmission from the broader spectral band, the fewer optical elements the better in order to reduce ghost images and improve system noise equivalent temperature difference (NETD).